How to apply soil back-fill pressure to a pipe?

I have a case of simulating a GRP Pipe in a trench, with soil back-filled upon it, 1m deep from ground surface to top to pipe.

My problem is, I am going to apply pressure (caused by soil) to outside surface of pipe, but I don't know the equation of this non-linear pressure. or , is there better way to apply soil pressure upon a cylinder surface?